Imaging devices with front face illumination generally comprise a set of pixels each having a photosensitive semiconductor region disposed under an integrated optical filter. Groups of pixels with red, green and blue filters may notably be used, in such a manner as to form Bayer patterns well known to those skilled in the art.
Optical filters designed to only let through a single color generally comprise an organic filter colored by pigments allowing infrared to pass. It is generally advisable to combine them with an additional infrared filter for imaging applications in the visible range.
Colored filters also have the drawback of not being sufficiently robust, and of not being able to be used for the infrared wavelengths.
Furthermore, a multicolor imager requires a number of photolithographic steps proportional to the number of types of colored filters to be formed. These filters also have the drawback of being degraded when they are exposed to temperatures in excess of 200° C.
On the other hand, interference filters (known as multilayer filters) also allow image sensors to be formed without being limited to the visible wavelengths, and they can therefore be used for applications outside of the visible range (UV or infrared). Their multilayer structure however makes them expensive and more difficult to integrate owing to the large number of deposition steps needed for their fabrication and to the total thickness of the stack, which can be greater than one micron.
Alternatives to colored filters using pigments and to multilayer filters have therefore been provided.
Optical filters are known from the prior art comprising one or more metal layers in which patterns (holes or bumps) are formed having dimensions of the order of ten to a hundred nanometers. This type of structure is better known by the term plasmonic filter.
For this purpose, reference may be made to the document “Light in tiny holes” (C. Genet and T. W. Ebbesen, Nature 445, pages 39-46, 4 Jan. 2007, the disclosure of which is incorporated by reference) which describes the transmission of light by nanometer-sized holes.
Reference may also be made to the document “Structural Colors: From Plasmonic to Carbon Nanostructures” (Ting Xu et al, Small Volume 7, Issue 22, pages 3128-3136, Nov. 18, 2011, the disclosure of which is incorporated by reference) which describes filters comprising nanometer-sized metal patterns for filtering various colors, notably with periodic patterns.
Finally, reference may be made to the document United States Patent Application Publication No 2003/0103150 (the disclosure of which is incorporated by reference) which discloses metal filters of the type presented hereinbefore which are formed by the same fabrication steps as the tracks of the metallization levels of the integrated circuits comprising these filters. The filters described in this document therefore have the same thickness as the metallization levels, and they comprise the same metal material as the tracks of the metallization levels.
The metal filters obtained in the latter document may not be suitable for filtering the light in a satisfactory manner. Indeed, the metal tracks of metallization levels have a height which can be of the order of several hundreds of nanometers or of the order of a micrometer, and the formation of patterns is limited by the constraints of photolithography and of filling with metal (notably when methods known as “Damascene” are applied). Thus, the solution of the document US 2003/0103150 does not allow the dimensions of the patterns that will form filters to be precisely chosen.
Generally speaking, the metal elements of metallization levels are formed between layers of silicon nitride (SIN) or of silicon carbo-nitride (SICN). These layers of silicon nitride or of silicon carbo-nitride therefore end up, in the solution presented in the document US 2003/0103150, on either side of the metal filters, a fact which is detrimental to the optical properties of the metal filters.